1. Field of the Invention
The present invention relates to a wireless communication apparatus and a receiving method.
2. Description of the Related Art
With an increase in frequency bandwidth used for communication to speed up wireless communication, it is difficult to neglect the propagation delay time differences between multipath channels. In an environment in which signals with different propagation delay times arrive, waveform distortion due to intersymbol interference is a large factor causing a deterioration in communication quality. In such an environment, an orthogonal frequency division multiplexing (to be referred to as OFDM hereinafter) scheme is known as a scheme which can compensate for waveform distortion due to intersymbol interference even if signals with different propagation delay times are received.
The OFDM transmission scheme uses complex signals, and hence needs to use a quadrature modulator and a quadrature demodulator for a transmitter and a receiver, respectively. In order to accurately generate an in-phase signal and a quadrature signal at the time of transmission, it is necessary to generate 90° out-of-phase signals. In order to accurately extract an in-phase component and a quadrature component at the time of reception, it is necessary to generate 90° out-of-phase local signals and multiply the reception signals by them. When gains are to be applied to an in-phase signal and a quadrature signal by using a filter or an amplifier, it is necessary to apply equal gains to the in-phase signal and the quadrature signal. In addition, when an in-phase signal and a quadrature signal are to be converted into digital signals by using A/D converters, any different delays must not be produced. In general, however, an in-phase component and a quadrature phase differ in amplitude, a phase error occurs in a 90° phase shifter, and a delay time difference occurs between the A/D converters which convert the in-phase component signal and the quadrature component signal into digital signals. Such imperfection in analog circuits is called IQ imbalance.
Assume that IQ imbalance has occurred. In this case, signals of two subcarriers, of a plurality of subcarriers of an OFDM signal, which are located at symmetrical positions with respect to the center frequency on the frequency axis (of the two subcarriers, the subcarrier in a frequency band higher than the center frequency and the subcarrier in a frequency band lower than the center frequency will sometime be called an “upper sideband subcarrier” and a “lower sideband subcarrier”, respectively) interfere with each other, and hence the transmission performance is greatly limited. There has been proposed a technique for estimating the amount of interference between upper and lower subcarriers in such an environment, and determining a signal by using two signals received with the upper and lower subcarriers (see, for example, reference 1: JP-A 2001-119364 (KOKAI)).
According to the prior art (reference 1) described above, it is also necessary to estimate the channel responses of interference signals with respect to symmetrical subcarriers caused by IQ imbalance and to transmit a channel estimation known signal having a length twice that generally required. In addition, as a channel estimation known signal, a signal sequence which can separate a desired signal from an interference component from symmetrical subcarriers must be transmitted, and hence this technique cannot be applied to an arbitrary system. Furthermore, since the prior art estimates a state including a channel response instead of distortion itself due to IQ imbalance, it is necessary to estimate a distortion component for each frame when signals must be received from an indefinite number of terminals or channel variations cannot be neglected.
In addition, although the prior art can correct IQ imbalance in a reception unit, it cannot correct distortion in a transmission unit.
The present invention has therefore been made in consideration of the above problems, and has as its object to provide a wireless communication apparatus including a reception unit which can accurately and easily estimate and correct phase distortion and amplitude distortion caused by quadrature demodulation.
It is another object of the present invention to provide a wireless communication apparatus including a reception unit which can accurately and easily estimate and correct phase distortion and amplitude distortion caused by quadrature demodulation and a transmission unit which can accurately and easily estimate and correct phase distortion and amplitude distortion caused by quadrature modulation.